1. Field of the Invention
The present invention relates to household refrigerators and more particularly to a temperature control system for a single evaporator, single fan type combination refrigerator including independent freezer compartment and fresh food compartment temperature controls.
2. Description of the Prior Art
Combination refrigerators of the "frost free" type including a single evaporator and a single fan for circulating air from the freezer and the fresh food compartments over the evaporator are well known. Examples are disclosed in U.S. Pat. No. 3,126,177--Schumacher and in U.S. Pat. No. 3,320,761--Gelbard. In such refrigerators, a major portion (approximately 90%) of the refrigerated air from the evaporator is directed through a passageway into the freezer compartment while a smaller portion (approximately 10%) is directed through a duct into the fresh food compartment. Two user-operable temperature control members are provided. One control member is for setting a temperature to be maintained in the fresh food compartment and is typically called either the "fresh food control" or "cold control." The fresh food control dial has graduations from "1" through "9," "1" indicating the warmest temperature and 9 indicating the coldest temperature to be set. The other control member is primarily for determining a preset temperature to be maintained in the freezer compartment and is typically called the "freezer control." The freezer control has graduations from A through E, with E being the coldest position.
The fresh food control is operatively connected to a thermostatic control which senses either fresh food compartment air temperature or a mixture of compartment air and incoming refrigerated air from the evaporator and thermostatically maintains the fresh food compartment temperature near the desired temperature by periodically energizing and de-energizing the refrigeration compressor, and thus the evaporator, in a conventional manner. Usually, the fan is energized and de-energized along with the evaporator. The thermostatic control causes energization of the evaporator when the fresh food compartment temperature exceeds the temperature setting of the fresh food control and causes de-energization of the evaporator when the fresh food compartment temperature is less than the temperature setting of the fresh food control.
The freezer control is connected in simple fashion to an air flow damper positioned in the duct which carries refrigerated air from the evaporator chamber to the fresh food compartment. In operation, as the freezer control is moved toward E, or the coldest position, the damper is closed more, reducing the amount of refrigerated air flowing into the fresh food compartment. Since the temperature in the fresh food compartment is thermostatically controlled, the compressor, under control of the thermostatic control simply runs longer or more often to satisfy the requirements of the fresh food compartment. When the compressor and evaporator run more, more refrigerated air flows into the freezer compartment for a longer period of time and the freezer compartment gets colder. Conversely, as the freezer control is moved toward A, or the warmest position, the damper is opened more, allowing more refrigerated air from the evaporator chamber to flow through the duct into the fresh food compartment. This causes the compressor and the evaporator to be energized less frequently or for shorter periods of time to satisfy the cooling requirements of the fresh food compartment. Since the temperature in the freezer compartment is directly related to the percentage of compressor and evaporator "on" time, the temperature in the freezer compartment decreases.
To summarize the above, in prior art refrigerators the temperature in the fresh food compartment is thermostatically controlled by energizing the compressor and evaporator in response to the cooling requirements of the fresh food compartment. Being under actual thermostatic control, the temperature is maintained quite efficiently at approximately the desired temperature. The temperature in the freezer compartment is not thermostatically controlled, but rather is controlled by varying the flow of refrigerated air from the evaporator chamber to the fresh food compartment, thereby forcing the compressor and evaporator to run for either longer or shorter periods of time to satisfy the requirements of the fresh food compartment, indirectly affecting the temperature in the freezer compartment.
Temperature control systems of the above-described type, while inexpensive and relatively effective, have the disadvantage that the fresh food and freezer controls do not exert truly independent control over the temperatures of the two compartments. The interaction between the temperature controls contributes significantly to customer dissatisfaction and costly complaints. In actual operation, the fresh food control, in addition to desirably setting a temperature to be thermostatically maintained in the fresh food compartment, undesirably affects the temperature of the freezer compartment. This undesirable effect is a direct result of the fact that, as the setting of the fresh food control is varied, in order to satisfy the cooling requirements of the fresh food compartment as determined by the fresh food control setting, the percentage of compressor and evaporator run time also varies. For example, if the compressor runs longer to maintain the fresh food compartment at a lower desired temperature, the freezer temperature also is lowered. The freezer control actually operates as a temperature differential control to maintain the freezer compartment temperature at a given temperature below the fresh food compartment temperature, the given temperature being determined by the setting of the freezer control. If the fresh food control setting is not disturbed, then the freezer control actually does control the temperature in the freezer. However, if the fresh food control setting is changed, with no change in the freezer control setting, the temperature differential between the two compartments is approximately maintained and the temperature in the freezer compartment undesirably goes up or down, depending upon the desired temperature change in the fresh food compartment.
Control interaction in the opposite direction, that is, fresh food compartment temperature variations as a result of changes in the setting of the freezer control, are not a significant problem because fresh food compartment temperature is substantially thermostatically maintained.
A further disadvantage which follows from the basic disadvantage of control interaction is that it is impossible to calibrate the freezer control directly in temperature. Any calibration of the freezer control directly in temperature would be valid only for a particular setting of the fresh food control.
Despite these disadvantages, the above-described prior art system enjoys wide use due to its relative simplicity and low cost. This points up the strict requirement that any improved system, to be practical, must also be relatively simple and low in cost.
A simple approach to the problem would be explaining to the user of the refrigerator the need to readjust the freezer control every time the setting of the fresh food control is changed. An astute user could adjust the controls to maintain the temperatures he desired in both compartments. The labeling of the "fresh food control" as a "cold control" in some refrigerator models is a step in this direction. However, there are problems in such an approach. A refrigerator with controls which appear complex to operate might be more difficult to sell. In the same vein, a detailed explanation might only serve to point out to a potential customer just how much undesirable control interaction there is. Further, many users either would not fully understand an explanation or would simply choose to ignore it.
It is known to mechanically overcome the above-described interactive control problem by thermostatically adjusting the damper in the duct carrying refrigerated air from the evaporator chamber to the fresh food compartment. In such a system, employing what is termed a "thermal damper," the freezer control does not control the damper directly, but rather is connected to a thermostatic control which senses the temperature in the freezer compartment. The control adjusts the damper opening in response to both the freezer compartment temperature and the control setting to thermostatically maintain the freezer compartment temperature. Although such a system works well, it suffers the disadvantage of added complexity with attendant higher cost and greater possibility of failure in use.
Another known way to achieve truly independent temperature control for the fresh food and freezer compartment is to provide separate thermostatically controlled fans for directing refrigerated air from the evaporator chamber to each of the compartments. Each of the fans is controlled in response to a thermostatic control located in the corresponding compartment. Such a system is disclosed in U.S. Pat. No. 3,005,321--Devery. While this system also should effectively provide independent control of the temperatures in the two compartments, it too suffers the disadvantage of complexity. Further, it would require extensive changes to existing refrigerator designs to implement it.
The above-mentioned copending Webb and Hester application Ser. No. 646,167 discloses and claims a generic refrigerator temperature control system which overcomes the problem of control interaction to provide truly independent control over freezer and fresh food compartment temperature. That system conventionally includes apparatus which varies airflow through the duct as a direct function of the setting of the freezer control. Additionally, in order to compensate for undesired changes in freezer temperature which would otherwise result when the setting of the fresh food control is changed, the variable airflow apparatus has an input ganged to the fresh food control and varies airflow as an inverse function of the setting of the fresh food control.
The present invention provides specific embodiments of variable airflow apparatus which controls duct airflow as the desired function of the settings of the two control members.